In the military, officer promotions are Darwinian in that there are only so many "slots" available for the next rank. You and all your peers which are able to be promoted will have their promotion packages reviewed and ranked by a board of officers. If you make the cut, you will be promoted. Generally your first promotion or two will be fairly automatic unless you've done something horribly wrong. Once you make Captain (O-3) however, things get MUCH more difficult. You have 3 chances to get promoted, you you blow it all three times, you will be discharged from the military.
You may want to also check out the Air National Guard (Guard = state; Reserves = federal).
Dave